Chemical cogeneration processes involve the reaction of a fuel with an oxidant to produce electricity and byproducts. These processes are sometimes also referred to as fuel cell processes. Such processes utilize electrochemical reactor cells to produce power and valuable chemicals. In these processes, the solid electrolyte cell functions as a fuel cell and as a chemical reactor. Fuel cells convert chemical energy into electricity with no intermediate combustion cycle. Consequently, their thermodynamic efficiency compares favorably with thermal power generation. Fuel cells convert inexpensive chemicals such as H.sub.2, CO and O.sub.2 into low-value products such as H.sub.2 O and CO.sub.2 while generating electrical power. However, there are several important industrial reactions, such as the conversion of H.sub.2 S to SO.sub.2 and of NH.sub.3 to NO, that have free energy (.DELTA.G) values comparable to that of N.sub.2 oxidation.
Gur et al. in U.S. Pat. No. 5,364,506 disclose an electrochemical reactor for partially oxidizing methane and cogenerating electrical energy. A solid-state ionic reactor is described in which a solid electrolyte is provided with a cathode and a perovskite type anode having a wide range of oxygen nonstoichiometry. The cell generates electrical energy as a result of the chemical potential difference brought about by the catalytic oxidation of methane at the anode with oxygen that chemically diffuses from the cathode through the solid-state ionic conductor.
Pujare et al. in U.S. Pat. No. 4,997,725 disclose a solid oxide fuel cell and process for direct conversion of natural gas into DC electricity concurrently with the electrocatalytic partial oxidation of methane to C.sub.2 hydrocarbon species C.sub.2 H.sub.4, C.sub.2 H.sub.6, and minor amounts of C.sub.2 H.sub.2. They disclose a solid oxide fuel cell comprising a metallic oxide oxygen reducing electronic and oxygen vacancy conducting perovskite cathode in contact on one side with an oxygen vacancy conducting solid electrolyte having high oxygen ion conductivity at fuel cell operation temperatures. An anode contacts the other side of the solid electrolyte and comprises a metallic oxide oxygen ion conducting perovskite layer contacting the solid electrolyte. A rare earth metallic oxide layer contacts the opposite side of the anode metallic oxide perovskite layer and is capable of dimerization of methane to predominately C.sub.2 products. Their process includes passing oxygen containing gas in contact with the outside surface of the metallic oxide oxygen reducing electronic and oxygen vacancy conducting perovskite cathode forming oxygen ion. The formed oxygen ion is passed from the cathode to and through an oxygen vacancy conducting solid electrolyte having high oxygen ion conductivity at fuel cell operating temperatures to the anode contacting the other side of the solid electrolyte. The metallic oxide oxygen ion conducting perovskite anode layer is in contact with the solid electrolyte on one side and is also in contact with methane on the other side. The anode oxidatively dimerizes methane to C.sub.2 species which are predominately C.sub.2 H.sub.4. The C.sub.2 species and electrons are withdrawn from the anode region.
Mazanec et al. in U.S. Pat. No. 4,793,904 describe an electrocatalytic process for producing synthesis gas from light hydrocarbons such as methane or natural gas. An electrochemical cell is provided which comprises a solid electrolyte having a first surface coated with conductive metal, metal oxide or mixtures thereof capable of facilitating the reduction of oxygen to oxygen ions; and a second surface coated with conductive metal, metal oxide or mixtures thereof, provided that both coatings arc stable at the operating temperatures. The cell is heated to a temperature of at least 1000.degree. C. An oxygen-containing gas is passed in contact with the first conductive coating and methane, natural gas or other light hydrocarbons are passed in contact with the second conductive coating. Lastly, a synthesis gas is recovered.
Krist et al. in U.S. Pat. No. 5,064,733 describe a process for concurrent gas phase electrocatalytic oxidative dimerization of methane at the anode on one side of a solid electrolyte and reduction of carbon dioxide to gaseous hydrocarbons at a cathode on the other side of the solid electrolyte. The process comprises passing methane containing gas in contact with a rare earth metallic oxide anode layer of an anode comprising a rare earth metallic oxide anode layer in contact with one side of an ionic and electronic conducting metallic oxide perovskite anode layer in contact with one side of a solid electrolyte on its opposite side. The anode catalytically oxidatively dimerizes the methane to C.sub.2 species, transferring through the solid electrolyte an ionic species selected from the group consisting of an oxygen ion species from the cathode to the anode and a proton mediating ion from the anode to the cathode at cell operating temperatures. Carbon dioxide containing gas is passed in contact with one side of an ionic and electronic conducting metal cathode electrocatalyst capable of providing adsorption sites for carbon dioxide and chemisorbed and Faradaically generated hydrogen species in proximity to the adsorbed carbon dioxide and capable of catalytic reduction of the carbon dioxide to predominantly C.sub.2 species. The produced C.sub.2 species is withdrawn from the region of the anode and the cathode.
Many of these processes utilize fuel cells which have designs that face problems in thermal cycling. In particular, mechanical stresses result from imperfectly matched thermal expansion coefficients causing delamination. In addition, operation at high temperatures leads to electrode sintering, interfacial reactions and migration of species which cause a significant voltage degradation over time. Moreover, because of the high operation temperatures, there is a need for large, costly heat exchange equipment.